Additive manufacturing or rapid prototyping methods for producing three-dimensional components are well known in the art (see for example U.S. Pat. No. 4,863,538-Deckard). There are various known methods of additive manufacturing including consolidation of powder materials and curing of polymeric resins. This invention relates to methods that involve powders. Such methods involve a layer-by-layer consolidation of powder material using a focused energy beam, such as a laser beam or an electron beam. Initially, the use of such freeform fabrication processes was restricted to the production of prototypes by sintering together layers of powder particles. Recent advances in technology, however, have meant that fully dense, high integrity components can be manufactured by freeform fabrication of components.
In a typical selective laser sintering (SLS) or selective laser melting (SLM) process, a thin layer of powder is deposited over a build area or powder bed within a SLS or SLM apparatus. A focused laser beam is scanned across portions of the powder layer that correspond to a cross-section of the three-dimensional article being constructed such that the powder at the points where the laser scans is consolidated either by sintering or by fusion. The cross-section is typically generated from a 3-D description of the component generated by scanning an original component or from computer-aided design (CAD) data.
After consolidation of a layer, the build surface is lowered by the thickness of the newly consolidated layer and a further layer of powder is spread over the surface. Again, the surface is irradiated with a laser beam in portions of the layer that correspond to a cross-section of the three-dimensional article, the newly consolidated layer being joined to the initial consolidated layer. This process is repeated until the component is completed.
In order to consistently manufacture an object with the desired structural properties it is important to maintain a controlled atmosphere, often a low oxygen content atmosphere, in the vicinity of the build surface during manufacture. Existing equipment achieves this by flooding a build chamber, which surrounds the build surface, with an inert gas. This inert gas is supplied at a slight overpressure compared with atmospheric pressure and allows the oxygen content of the atmosphere within the build chamber to be substantially reduced to about one percent oxygen. A low oxygen atmosphere in the region of the build plate allows the powder to be heated, and where necessary melted, without undergoing excessive undesirable oxidation reactions.
FIG. 1 illustrates a typical arrangement of a build chamber 20 in a prior art rapid prototyping system 10. The build chamber 20 defines a space 25 above a lowerable build platform 30 and is constructed with sufficient integrity to be flooded with inert gas at a slightly elevated pressure. The build chamber 20 contains a powder dispensing and coating apparatus 40 for spreading powder 45 over the surface of the build platform and a window in an upper wall of the chamber 50 allows optical access to a laser beam 55 for irradiating powder spread at a build surface 35. Thus, any sintering and/or melting operations at the point of interaction between the laser beam and powder 56 occur in a low oxygen atmosphere.
The build chamber is supplied with a number of different gas transport circuits 60, 70, 80, each gas transport circuit having its own pump 61, 71, 81, and its own filter 62, 72, 82.
The build platform is arranged to be lowerable within the bore 95 of a build cylinder 90, which allows the build surface 35 to remain in substantially the same position within the machine while an object 100 is built up from successive powder layers. The build platform is typically lowered by a piston mechanism 110 and its edge 32 incorporates a seal 33 that engages with the bore 95 of the build cylinder 90 to prevent egress of powder 45 from the build cylinder.
This prior art system presents a number of disadvantages. Overpressure in the build chamber tends to force powder through the seals between the build platform and the bore of the build cylinder (illustrated by arrows in FIG. 1). This results in loose powder becoming deposited within the apparatus, from where it must be cleaned, and may also compromise the integrity of the seal and allows oxygen to enter the build chamber during manufacture.
Gas transport devices such as pumps, valves and pipes are all prone to leaks, particularly at connection points. Any oxygen leaking into such a point in the gas transport system can compromise the integrity of the low oxygen atmosphere in the build chamber.